Field of the Invention
This invention relates to an arrangement of a jackup platform's leg chords, elevating system, and leg guides that work in conjunction with a hull-to-legs load transfer device, to support the weight of the hull and storm induced forces between the legs and the hull of the jackup platform. The elevating system arrangement has climbing pinion gear unit support housings that contain both gear units on one side of a trussed leg chord and rollers on the opposite side. The housings are connected to the hull of the jackup platform with pinned links that allow the housings to move laterally with respect to the hull. This ability to move laterally, allows the jack housings to be guided to the legs and to also move with the legs, as they move within the constraints of the leg guides, that are in the openings in the hull, through which the legs pass. The load transfer device is an apparatus that consists of rectangular load blocks, with protruding lugs, that interact with toothed gear racks on the jackup's legs, to transfer hull weight and storm induced forces from the hull to the legs.
Self-elevating type mobile offshore platforms, commonly referred to as "jackups", have been used for oil or gas well drilling, work platforms, oil or gas production platforms, and many other uses. These jackups usually consist of a barge shaped hull, supported by three or more trussed legs which usually extend vertically through openings in the hull. The trussed legs are usually fitted with vertically extending toothed gear racks on the chords of the legs and the hull is usually fitted with elevating gear units, commonly referred to as "jacks", that engage with the gear racks to raise and lower the legs when the jackup is afloat and to raise and lower the hull when the legs have penetrated the ocean floor.
For normal operations, when putting a jackup on an operating location, the legs are lowered to the ocean floor with the jacks and jacking continues until soil resistance to penetration of the legs causes the hull to lift out of the water a few feet. Additional soil resistance is usually developed to simulate the largest reaction between the legs and the ocean floor that may be anticipated while at that location. This is normally done by pumping sea water into ballast compartments of the hull. After developing this additional soil resistance, the hull is then elevated to the desired elevation, which is at least high enough to assure that the crest of the largest anticipated waves will be below the bottom of the hull.
While elevated in this operating position, jackups may be subjected to large loads from storm winds, waves and currents. These loads induce large interacting forces and moments between the hull and the legs of jackups.
The elevating gear units of a jackup, commonly referred to as "jacks", are usually mounted in housings that are located radially out from the center of each leg chord and extend vertically up from a location above the top deck of the hull. The gear units are normally mounted one above the other in the housings. Usually there are two levels of leg guides which keep the legs relatively perpendicular to the hull bottom. With this arrangement, the jacks resist all vertical interaction forces between the hull and the legs and the jacks work together with the leg guides to resist the storm induced moment between the hull and the legs.
One common arrangement of gear racks and jacks is to orient the gear racks and climbing pinions of the jacks radially out from the center of each leg. With this arrangement there is one gear rack per leg chord and one vertical row of jacks that interact with the single gear rack at each chord. When a climbing pinion of a jack interacts with a vertical gear rack the resultant force applied to the gear rack is relatively perpendicular to the contact face of the gear rack teeth. With this arrangement, the horizontal components of the forces applied to the gear racks, by the jack pinions, induce large forces into the leg braces, that are located nearest to a vertical position that is between the pinions and leg chords. These forces have a significant effect on the required size and strength of the bracing members. Since for different operating water depths, the jack pinions are aligned with different vertical positions on the legs, the required strength of most all the leg bracing is affected by the horizontal component of the forces applied to the gear racks by the jack pinions.
Another common arrangement of gear racks and jacks is to have opposed pairs of gear racks on each leg chord and opposed climbing pinions engaged with the opposed gear racks. With this arrangement the horizontal components of the pinions counteract each other through the leg chord instead of through the leg bracing. Although this arrangement prevents inducing the horizontal components of the pinion reactions into the leg braces between the leg chords, it does require two gear racks per leg chord. This double gear rack arrangement results in increased leg weight and leg construction cost.
When a jackup's hull is elevated above the water surface and the legs are subjected to storm loads, the magnitude of these loads is proportional to the projected area in the direction of the storm. The magnitude of these loads is also very sensitive to the shape of the individual members. Two gear racks on a leg chord has more projected area and a much worse shape factor for storm forces than a similar chord with only one gear rack. In general, this means that jackups, with opposed gear racks and jack pinions, have the disadvantage of having to resist higher magnitudes of storm forces when compared to jackups with similar shaped leg chords with only a single gear rack.
There are two basic types of jack housings that are commonly used on jackups. One type is where the jack housings is an integral part of the hull. Jackups with this type of jack housing is said to have "fixed jacks". Fixed jacks are relatively stiff, which causes a significantly large part of the interaction moments between the legs and the hull to be transferred through the jacks rather than through the leg guides. These storm induced moment reactions to the jacks, when combined with the gravity reactions to the jacks, may require the need for more jack units to adequately resist these reactions, than would be necessary for elevating the hull on the legs when making location moves. These additional jack units can significantly increase the cost of a jackup.
With fixed jacks the gear rack engagement pinions move laterally in all directions, with respect to the gear racks, as the legs move within the constraints of the leg guides. This may cause increased wear on the gear racks. To minimize this movement and wear, it is necessary to have very small clearances between the leg chords and the leg guides. In order to maintain small clearances, it is necessary to fabricate the legs very accurately to avoid looseness or binding in the leg guides. This requirement for high tolerance leg fabrication, increases the cost of leg construction, when compared with jackups that do not have fixed jacks.
The other type of jack housing that is commonly in use is one that reacts against the hull but is not physically connected to the hull. Jackups with jack housings of this type are said to have "floating jacks". For jackups with floating jacks, raising or lowering the legs, while afloat, will cause the bottoms of the jack housings to bear vertically against resilient pads which bear against the hull. For jackups with floating jacks, raising or lowering the hull, while the hull is supported above the water by the legs, will cause the tops of the jack housings to bear vertically against resilient pads which bear against ridged structural framework, that is attached to the hull. Jack housings of floating jacks, are guided to the leg chords and move with the legs as they move laterally within the constraints of the leg guides. As the jack housings move laterally, with respect to the hull, the resilient pads are flexible enough to laterally distort elastically.
For jackups with floating jacks, leg guide clearances do not need to be as small as for jackups with fixed jacks, because the lateral movement, of the legs, in the leg guides does not affect the meshing of the jack pinions and gear racks. With more clearance in the leg guides, the tolerances for leg fabrication can be relaxed. This can reduce leg fabrication costs, for jackups with floating jacks, when compared with jackups that have fixed jacks. The resilient pads are relatively soft which causes most of the interaction moment between the legs and the hull to be transferred through the leg guides rather than through the jacks. The result of this is less storm induced reactions to the jacks, which may result in a reduced number of jacks required to resist these reactions. This can significantly reduce the cost of a jackup, when compared with fixed jacks.
Since most of the interactive moments between the legs and the hull are taken by the leg guides, for jackups with floating jacks, the horizontal guide reactions between the leg chords and the leg guides are much higher that for jackups with fixed jacks. These upper and lower guide reactions create large axial forces in the leg braces that are located vertically between the upper and lower guides. Since for different operating water depths, different braces of the legs are located between the upper and lower guides, the required strength of most all leg braces are affected by these high guide reactions.
One disadvantage for jackups with floating jacks is the increased initial cost of the jackup due to the purchasing of the resilient pads. Another disadvantage is the cost of replacing these resilient pads while the jackup is in service. These resilient pads are usually made of natural rubber which deteriorates with age and the pads have to be replaced periodically.
When a jackup is elevated above the water, the independent legs may have differing amounts of penetration into the ocean floor. Because of this unequal penetration, and also because of unlevel ocean floors, it may not be possible to elevate the hull to a vertical position that will align the lower leg guides of the hull with brace-to-chord intersection nodes at each of the legs. When storm forces cause the leg guides to react laterally against the leg chords, at a location between the nodes, the reaction forces may cause excessive bending moments in the leg chords. The stresses, in the leg chords, that are caused by these bending moments, combines with the stresses due to the axial force in the leg chords. The highest axial stresses, in the leg chords, is normally located in way of the lower guides of the hull. This is also the location of high leg chord bending moments due to lower guide forces. The combination of axial stress in the leg chord, with the bending stresses caused by the lower guide forces, can require substantially higher strength for the leg chords than would be required if the lower guide forces were always located at a leg node. This combined stress requirement exists for jackups with either fixed jacks or floating jacks. It is more severe for jackups with floating jacks than for jackups with fixed jacks. This is because jackups with floating jacks, when compared with jackups that have fixed jacks, have a larger portion of the interaction moment between the legs and the hull taken by the leg guides, rather than by the jacks.
The various problems described above are represented in the art. Attempted solutions are presented in U.S. Pat. Nos. 3,343,371 Heitkamp; 4,269,543 Goldman et al.; 4,389,140 Bordes; 4,538,938 Grzelka et al.; 4,627,768 Thomas et al.; 5,092,712 Goldman et al.; 5,139,366 Choate et al.; 5,188,484 White; 5,486,069 Breeden; 5,611,645 Breeden; and 5,622,452 Goldman. Although the present invention provides solutions to problems not found or suggested in the listed patents, each of the cited references is hereby incorporated by reference for all disclosed therein.
As designers searched for solutions to the problems associated with the interactions of the legs and the hull of independent leg jackups, the development of a type of locking system that is now commonly known as "rack chocks" was developed. A rack chock consists of a section of gear rack, with the same tooth profile as the gear rack, on the leg chords, and various mechanisms to manipulate and secure the section of gear rack in a position where the matching profiles of the gear rack teeth of the leg chord and the gear rack teeth of the rack chocks are intermeshed. Once intermeshed and tightly secured, the weight of the hull and it's contents can be transferred from the jacks, of a jackup, to it's rack chocks. Then the upper, lower, and end faces, of the gear rack teeth and rack chock teeth, can interact to transfer combinations horizontal and vertical forces caused by the weight of the hull and the storm induced interacting forces between the legs and the hull.
The usual arrangement, for jackups with rack chocks, is for the rack chocks to be below the jacks and either above or just below the top deck of the hull. The upper leg guides are usually located above the jacks and the lower leg guides are usually located at the bottom of the hull. With a jackup's hull elevated above the water, and with it's rack chocks engaged, a storm can apply forces to the jackup that will cause the hull to deflect laterally and the legs to bend such that there will be differing amounts of relative lateral deflection between the hull and the portion of the legs that extend below the rack chocks. This relative deflection increases from zero at the rack chocks to a maximum at the bottom of the legs. When this happens, if the lower guides are some distance below the rack chocks and have small clearances to ensure that the legs are held in good alignment, for proper meshing of the gear teeth of the jacks with the gear racks on the leg chords, the lower guides will react against the legs preventing any relative deflection between the hull and the legs in the horizonal plane of the lower guides. These reactions, as previously explained, can induce increased axial forces in the braces of the leg and bending moments in the leg chords, adversely affecting their design. If the lower guides have very loose clearances to avoid these interaction forces at the lower guides, the legs cannot be held in good alignment with the hull and the jack pinions, when operating the jacks. The alignment, with this loose guide arrangement, will be dependent on the meshing of the jack pinions with the gear racks, and this is undesirable. Without close clearance lower guides, environmental forces that may exist while operating the jacks to make location moves, will induce interaction forces between the jack pinions and the gear racks that would normally be prevented by small clearance lower guides. Not only is leg to hull alignment affected, but there will likely be more rapid wearing of the gear racks by the jack pinions.
Rack chocks have been found to be very difficult to disengage. This is because of the multiple directions of interacting forces that the meshed gear rack and rack chock teeth can take. To disengage the rack chocks, it is necessary to operate the jacks to transfer the load from the rack chocks to the jacks. Because of the matching tooth profiles of the gear racks and the rack chocks, the load directions for the interaction forces between the gear racks and the rack chocks can reverse as the jacks transfer the weight of the hull and its contents. When this happens, the jacks will be carrying more than the weight of the hull and it's contents and the rack chocks will be loaded, in reverse, with the difference between the load on the jacks and the weight of the hull and its contents. To stop jacking at the precise moment when there is no load on the rack chocks, or when the load is small enough to allow the rack chocks to be retracted, is very difficult and time consuming. It could involve repeatedly reversing the jacks to remove the rack chocks, one leg chord at a time.
Gear racks and rack chocks are normally flame cut to the same profile. Because of this, there will be some misfit upon engagement of the rack chocks with the gear rack. This misfit can led to unequal load distributions between the individual gear rack and rack chock teeth. Depending on the degree of misfit, it may require local yielding at individual teeth before load sharing of all of the rack chock teeth can take place. Local yielding of rack chock teeth, caused by storm loads during engagement with gear rack teeth, that are out of tolerance, could be the cause of additional misfits when the rack chocks are engaged with the gear racks at other vertical locations on the legs.